In recent years, silicon carbide crystals have drawn considerable attention in the industry because of their advantageous properties as a semiconductor material, such as a large band gap, high saturated electron drift velocity, and high thermal conductivity. Single crystalline substrates made of 6H or 4H hexagonal silicon carbide are commercially available and used. Among them, a cubic silicon carbide has been particularly expected for use in a semiconductor device with high speed and high power operation. However, the cubic silicon carbide is very difficult to be developed into a large single crystal usable as a substrate, and for this reason, a thin film heteroepitaxially grown on a single crystalline silicon or the like has been conventionally used.
Such silicon carbide crystals as above are usually formed by an atmospheric pressure CVD and the like method under a high temperature over 1300.degree. C., normally around 1500.degree. C., using a mixed gas of silane and propane as a source gas, and hydrogen as a carrier gas.
Nonetheless, the growth mechanism of a silicon carbide crystal has not yet been fully understood, and, therefore, an industrial technique for epitaxially growing a silicon carbide thin film with high reproducibility by controlling the relationship between an amount of a source gas to be supplied and a temperature of the substrate has not yet been established with full knowledge thereof. As a consequence, conventional production methods for a silicon carbide substrate have various drawbacks such as described below.
Firstly, since high temperatures as mentioned above are required for the crystal growth of silicon carbide, it has been difficult to grow a crystal in a selective area by using masking, and to carry out a nitrogen doping with high concentration. Specifically, since there is no suitable material for masking sufficiently resistant to such high temperatures, it is difficult to grow a crystal only in a predetermined region by patterning. In addition, silicon carbide is difficult to be subjected to a selective etching, and therefore it is difficult to form desired semiconductor devices and semiconductor circuits using silicon carbide. In addition, if a nitrogen doping is carried out under such a high temperature as above, the resulting film of the grown crystal is susceptible to roughness, and therefore a doping with a high concentration such as approximately more than 5.times.10.sup.18 /cm.sup.3 is difficult to attain. Furthermore, in a crystal growth at a high temperature, the decomposition, attaching to a surface of the substrate, re-evaporation and the like mechanisms of a supplied source gas are so complicated that it is rendered more difficult to, for example, epitaxially grow a silicon carbide thin film with a high reproducibility by controlling the relationship between the amount of the source gas to be supplied and the temperature of the substrate. It is noted that T. Kimoto et al. suggest on pp. 726-732 in the Journal of Applied Physics. Vol. 73, No. 2 (1993) a technique of forming a 6H silicon carbide crystal at a relatively low temperature by employing a silicon carbide substrate having an off-cut surface towards a [1120] direction of a {0001} face by using a step-flow growth and the like. However, even with this technique, it is required that the substrate be heated at approximately 1200.degree. C.
Secondly, a single crystal thin film composed of hexagonal or cubic silicon carbide with a good crystallinity is difficult to be epitaxially grown. In particular, when a cubic silicon carbide crystal is formed on a silicon substrate, a large lattice mismatch occurs and therefore a good crystallinity is difficult to obtain. In addition, when a cubic silicon carbide crystal is formed on a 6H hexagonal silicon carbide crystal, the resulting cubic silicon carbide crystal is apt to contain double positioning boundaries caused by the occurrence of twin. It is noted that the present inventors have disclosed in Japanese Unexamined Patent Publication No. 07-172997 a method for producing a cubic silicon carbide thin film having a (001) face by having silicon atoms present in excess of carbon atoms on the growth surface of a silicon carbide crystal, and a method for producing a cubic silicon carbide thin film having (111) surface or a hexagonal carbon silicon thin film having (0001) surface by having carbon atoms present in excess of silicon atoms on the growth surface of a silicon carbide crystal. However, even with these methods, it has been difficult to suppress the occurrence of twin certainly or drastically although a relatively good crystallinity can be obtained thereby.
Moreover, it has not yet been made possible to heteroepitaxially grow on a silicon carbide substrate a silicon carbide crystal having a different crystal system from a crystal system of the substrate. For example, in the case of a step-flow growth of a silicon carbide crystal on a hexagonal silicon carbide substrate, the resulting silicon carbide crystal is apt to retain a hexagonal structure because the crystal structure of the substrate tends to restrict the resulting crystal, and it is therefore difficult to heteroepitaxially grow a cubic carbon carbide on such a substrate.
In view of the above problems, it is an object of the present invention to provide a method for producing a silicon carbide substrate that it is capable of epitaxially growing a silicon carbide having a good crystallinity at a relatively low temperature, that a crystal growth in a selective region by masking and a high concentration nitrogen doping are easily carried out, and that silicon carbides each having a different crystal system are heteroepitaxially grown and stacked with a good interface therebetween.
It is another object of the present invention to provide a silicon carbide substrate produced by the above method.
It is further another object of the present invention to provide a semiconductor device capable of high-speed operation utilizing the above silicon carbide substrate.